The concept of this story is great, and I love all of the episodes. So, you're probably wondering why I haven't given this a full five stars:

The episodes are great, yes, but only individually. They aren't great together.

Some plot points get lost. The cliffhanger of episode one is Tegana getting poison for the water supply, which he will use to poison Marco and co as they trek through the Gobi Desert... Except he doesn't. He loses the poison in a sandstorm and ends up slashing the gourds instead, which makes the cliffhanger feel unecessary.

Additionally, every episode is a repetition of the same format, which, when watched together (I know, that's a modern problem) results in an experience which is, overall, rather boring. There's only so many 'TARDIS team try to get back to the TARDIS and are prevented. Tegana plots against everyone' loops you can take in one go.

There is a 30-minute version on The Beginning DVD. I don't own this, so I can't say how good it is, however I don't think this story could really lose that much of its runtime. An episode or two, sure. Five episodes? I don't think so.